Implement lint for obligations broken by never type fallback change

[WaffleLapkin]

This is the second (and probably last major?) lint required for the never type fallback change.

The idea is to check if the code errors with fallback = () and if it errors with fallback = ! and if it went from "ok" to "error", lint.

I'm not happy with the diagnostic, ideally we'd highlight what bound is the problem. But I'm really unsure how to do that (cc @jackh726, iirc you had some ideas?)

r? @compiler-errors

Thanks @BoxyUwU with helping with trait solver stuff when I was implementing the initial version of this lint.

Tracking:

• Tracking Issue for Rust 2024: Make ! fall back to ! #123748

[jackh726]

I'm not happy with the diagnostic, ideally we'd highlight what bound is the problem. But I'm really unsure how to do that

So, yeah, we talked about this. IIRC, what I said was basically like "we can't find diff two error sets" but as long as we only try to report ! fallback errors if () fallback errors are empty, that's probably fine...

We can chat on zulip about details here, but I would probably approach this by selecting a couple test cases where you want better diagnostics, then look at the errors that you get and what info is there, then figure out how to extract out that info. Then you can generalize across tests.

[WaffleLapkin]

Quite a few of existing tests get the warning, should I do something with it? The options I see are bless them (what I currently did), fix them and allow the warning.

[bors]

☔ The latest upstream changes (presumably #125313) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy